How Do You Track Altitude of Cloud Bases?

Question: I am a student pilot and have learned that the airport’s automated weather machine is prone to malfunctioning and unreliable, so we’ve been taught not to trust it. The winter is all low clouds, and sometimes the pattern is a little iffy. Is there an easy way to determine the base of the clouds beyond taking off and seeing if you have the room to fly the pattern?

Answer: There is a basic math formula:

• Take the surface temperature in Fahrenheit and subtract the dew point.
• Divide the result by 4.4, then multiply the answer by 1,000, and you will get the cloud base above ground level (agl). Most airport traffic patterns are approximately 1,000 feet agl.

So let’s say the temperature is 42 degrees F and the dew point is 20 degrees:

• 42-20 = 22, divided by 4.4 = 5
• Multiple that by 1,000 = Expect the cloud bases at 5,000 feet.

Keep in mind that when the temperature-dew point spread converges—for example, late in the day when the sun is on the way down—the clouds can drop and fog can form rather quickly. If the airport has automated weather, keep an ear on the temperature-dew point spread in addition to watching for an increase in clouds and their position relative to the pattern.

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